bonding: Improve syslog message at device creation time

When the bonding module is loaded, it creates bond0 by default.
Then, when attempting to create bond0, the following messages
are printed to syslog:

    kernel: bonding: bond0 is being created...
    kernel: bonding: Bond creation failed.

Which seems to indicate a problem, when in reality there is no
problem.  Since the actual error code is passed down from bond_create,
make use of it to print a bit less ominous message:

    kernel: bonding: bond0 is being created...
    kernel: bond0 already exists.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>